The problem is context, a common issue with tracking things with Google Trends in particular. Tracking programming language usage with it, for example, has been a nightmare ("ruby" and "python" having far too many meanings, but few write "ruby programming").
"high end" has more uses than "high-end." For example, "I bought a car at the high end of my budget." In that case, "high-end" wouldn't make sense. In "datacenters have been targeted by high-end criminals," however, "high-end" is a compound adjective.
Alternatively, you could drop the hyphen and/or form an entirely new word: "highend." The word "highend" doesn't seem to have caught on yet, though. I suspect that's because "upmarket" covers the same meaning already and is less susceptible to these morphological mishaps.
(On seeing what OS X had to suggest as a correction for "highend," it suggested both "high end" and "high-end.")